Marvel Television and ABC's
"Agents of SHIELD" is playing coy with its Season 2 mysteries in the same way it did with Season 1. A cryptic billboard popped up in Los Angeles (above) that features the bizarre alien scribbling both Agent Phil Coulson (Clark Gregg) and John Garrett (Bill Paxton) drew after consuming GH-325 at the end of Season 1.

The biggest mystery kicking off the beginning of Season 1 was "How is Coulson alive?" The big Season 2 mystery seems to be "What message is the GH-325 trying to send?" Some speculation is that the blue alien the GH-325 is drawn from is Kree, a species introduced in "Guardians of the Galaxy." If that's the case, "SHIELD" and "Guardians" are
ripe for a crossover.

Even though the effects of GH-325 are the focal point of the "Agents of SHIELD" Season 2 billboard, it's far from the only mystery Agents Coulson, May, Fitz, Simmons, Triplett and Skye are facing. The identity of Skye's father will be dealt with as
Kyle MacLachlan was cast in the role. Ward's fate and allegiance will (likely, hopefully) be determined, and it's still unclear if the Koenigs are Life Model Decoys. Read our full list of Season 2 burning questions (with some answers)
here.

Other newcomers to Season 2 include Lucy Lawless as SHIELD agent Isabel Hartley, Reed Diamond as HYDRA's Daniel Whitehall and Nick Blood as mercenary Lance Hart. Meanwhile Adrian Pasdar will reprise his role as Glenn Talbot, and the character Mockingbird
has not yet been cast
will be played by
Adrianne Palicki.